Evidence for emission and transfer of energy from excited edge sites of MgO smokes by photoluminescence experiments

New features are observed in photoluminescence spectra of MgO smokes produced in controlled conditions, an experiment inspired by previous measurements on cleaved MgO(1 0 0). Upon photoexcitation at 5.17-5.45 eV, most luminescence spectra observed can be decomposed into two bands, peaking at 3.82-3.86 eV (the highest energy ever recorded in such experiment) and 3.20 eV, respectively. It is assumed that the 3.82-3.86 eV band is related to the luminescence at four-coordinated (4C) defect-free edge sites and that the 3.20 eV band arises from the excitation transfer from 4C edge sites towards less-coordinated 3C sites. A minor contribution to the 3.20 eV band observed upon excitation with 4.6 eV photons is assigned to a direct excitation of 3C sites. Within these assumptions, water chemisorption-desorption is suggested to produce less-coordinated sites (the 3.20 eV band increases in intensity) at the expense of 4C sites (the 3.84 eV band decreases). The huge sensitivity of the 3.84 eV/3.20 eV branching ratio to the formation of defects on 4C edges is suggested to explain the lack of consensus about the photoluminescence of MgO.